EVAL-AD7274CB Analog Devices Inc, EVAL-AD7274CB Datasheet - Page 16

• Manufacturer Part Number: EVAL-AD7274CB
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD7274CB

• Lead Free Status / Rohs Status: Not Compliant

AD7273/AD7274
TYPICAL CONNECTION DIAGRAM
Figure 27 shows a typical connection diagram for the AD7273/
AD7274. An external reference must be applied to the ADC.
This reference can be in the range of 1.4 V to V
reference, such as the REF19x family or the ADR421, can be
used to supply the reference voltage to the AD7273/AD7274.
The conversion result is output in a 16-bit word with two leading
zeros followed by the 12-bit or 10-bit result. The 12-bit result from
the AD7274 is followed by two trailing zeros, and the 10-bit result
from the AD7273 is followed by four trailing zeros.
Table 7 provides some typical performance data with various
references under the same setup conditions for the AD7274.
Table 7. AD7274 Performance (Various Voltage Reference IC)
Voltage Reference
AD780 @ 2.5 V
AD780 @ 3 V
REF195
ANALOG INPUT
Figure 28 shows an equivalent circuit of the analog input
structure of the AD7273/AD7274. The two diodes, D1 and D2,
provide ESD protection for the analog inputs. Care must be
taken to ensure that the analog input signal never exceeds the
supply rails by more than 300 mV. Signals exceeding this value
cause these diodes to become forward biased and to start
conducting current into the substrate. These diodes can
conduct a maximum current of 10 mA without causing
irreversible damage to the part. Capacitor C1 in Figure 28 is
typically about 4 pF and can primarily be attributed to pin
capacitance. Resistor R1 is a lumped component made up of the
REF195
2.5V
10pF
Figure 27. AD7273/AD7274 Typical Connection Diagram
0V TO V
INPUT
0.1
μ
REF
F
V
V
AGND/DGND
IN
REF
AD7274 SNR Performance
1 MHz Input
71.3 dB
70.1 dB
70.9 dB
AD7273/
AD7274
4.6 mA
V
DD
SDATA
0.1
SCLK
INTERFACE
μ
CS
SERIAL
F
DD
10
. A precision
μ
F
μ
DSP/
3.6V
SUPPLY
C/
μ
P
Rev. 0 | Page 16 of 28
on resistance of a switch. This resistor is typically about 75 Ω.
Capacitor C2 is the ADC sampling capacitor and has a capacitance
of 32 pF typically. For ac applications, removing high frequency
components from the analog input signal is recommended by
using a band-pass filter on the relevant analog input pin. In
applications where harmonic distortion and signal-to-noise
ratio are critical, the analog input should be driven from a low
impedance source. Large source impedances significantly affect
the ac performance of the ADCs. This may necessitate the use
of an input buffer amplifier. The AD8021 op amp is compatible
with this device; however, the choice of the op amp is a function
of the particular application.
When no amplifier is used to drive the analog input, the source
impedance should be limited to a low value. The maximum source
impedance depends on the amount of THD that can be tolerated.
The THD increases as the source impedance increases and perfor-
mance degrades. Figure 14 shows a graph of the THD vs. the
analog input frequency for different source impedances when
using a supply voltage of 3 V and sampling at a rate of 3 MSPS.
DIGITAL INPUTS
The digital inputs applied to the AD7273/AD7274 are not
limited by the maximum ratings that limit the analog inputs.
Instead, the digital inputs can be applied at up to 6 V and are
not restricted by the V
example, if the AD7273/AD7274 were operated with a V
3 V, then 5 V logic levels could be used on the digital inputs.
However, it is important to note that the data output on SDATA
still has 3 V logic levels when V
SCLK and CS not being restricted by the V
that power supply sequencing issues are avoided. For example,
unlike with the analog inputs, with the digital inputs, if CS or
SCLK are applied before V
V
IN
4pF
C1
Figure 28. Equivalent Analog Input Circuit
V
DD
DD
D1
D2
+ 0.3 V limit of the analog inputs. For
CONVERSION PHASE–SWITCH OPEN
TRACK PHASE–SWITCH CLOSED
DD
, there is no risk of latch-up.
DD
= 3 V. Another advantage of
R1
C2
DD
+ 0.3 V limit is
DD
of